An electronic sensor usually includes one or more transducers connected, to control circuitry, which may be provided separately, e.g. as an ASIC. A sensor will usually include several transducers connected together again by bond wires, as e.g. a MEMS. Examples of applications for such sensors include vehicle navigation systems; handheld navigation systems, vehicle ESP (electronic stability control systems); vehicle airbag deployment; model helicopter stabilisation; agricultural spray boom stabilisation; satellite receiver stabilisation; UAV navigation and stability systems; and light aircraft secondary instrumentation. Some of these applications have more stringent requirements on fault detection than others.
Sensors can provide unreliable results if a bond wire between transducers, or between a transducer and the control circuit, breaks or is damaged. A wire bond failure can result in a sensor output that is plausible—i.e. appears to be within a specified operating range, but which is, in fact, erroneous.
For example, a sensor comprising four sense transducers connected to an amplifier would, if one transducer was lost due to e.g. a lead failure, have its output signal reduced by ¼ (25%). It is possible, however, for the output signal to vary under normal operation, by up to 32% over temperature and tolerances. It is difficult, therefore, to determine whether a reduction in output signal is indicative of loss of a transducer or just due to normal fluctuations.
Devices and techniques have been proposed to increase the accuracy and reliability of sensors but typically require additional complexity, including the injection of trace signals with demodulation techniques to reject the trace signal from the normal required signal and measurement of the trace signal to allow fault detection increasing circuit complexity, injection of trace signals under the command of the host system but does not provide continuous fault detection and is subject to periods of the signal being invalid during the applied test, failure detection circuits with a threshold limit that is adjusted with temperature to compensate for the variation of the normal level over temperature increasing circuit complexity and requiring knowledge of temperature variation and the use of additional transducers on the MEMS to allow comparison of the signal against a reference signal increasing the complexity of the MEMS and reducing the allowable area for the detection transducers therefore reducing sensor performance through degraded signal to noise ratios and sensitivities.